The invention relates to a crystalline form of the sodium salt of telmisartan, processes for preparing it and the use thereof for preparing a pharmaceutical composition.
Telmisartan is the nonproprietary name (NN, USAN and BAN) for 4xe2x80x2-[2-n-propyl-4-methyl-6-(1-methylbenzimidazol-2-yl)benzimidazol-1-ylmethyl]biphenyl-2-carboxylic acid. It has the following chemical structure 
and is known from European Patent EP 502 314 B1 and U.S. Pat. No. 5,591,762.
Telmisartan, and the physiologically acceptable salts thereof, have valuable pharmacological properties. Telmisartan is an angiotensin antagonist, particularly an angiotensin-II-antagonist which by virtue of its pharmacological properties may be used for example to treat hypertension and cardiac insufficiency, to treat ischaemic peripheral circulatory disorders, myocardial ischaemia (angina), to prevent the progression of cardiac insufficiency after myocardial infarct, to treat diabetic neuropathy, glaucoma, gastrointestinal diseases and bladder diseases. Other possible therapeutic applications can be found in EP 502314 B1 and U.S. Pat. No. 5,591,762.
A pharmaceutical formulation comprising telmisartan as the active ingredient, Micardis(copyright) (telmisartan) Tablets, is commercially available. Starting from the free acid of telmisartan, the preparation in the form in which telmisartan is marketed is produced by a complex spray drying process. Because of the limited solubility of the free acid, less complex methods of preparing an alternative preparation are difficult to achieve.
Thus, one objective of the present invention is to find a less complex and easier means for preparing a crystalline form of telmisartan that would be suitable for use in the preparation of pharmaceutical formulations.
It has to be borne in mind that generally the production of a composition containing a pharmaceutically active substance is based on various parameters which are linked to the nature of the active ingredient itself. Without being tied thereto, examples of these parameters are the stability of effect of the starting material under different environmental conditions, the stability during the manufacture of the pharmaceutical formulation and the stability in the final compositions of the pharmaceutical preparation. The pharmaceutically active substance used to prepare the abovementioned pharmaceutical compositions should be as pure as possible and its stability on long-term storage must be guaranteed under various environmental conditions. This is absolutely essential, in order to prevent pharmaceutical compositions being used which contain, in addition to the active substance proper, breakdown products thereof. In such a case the content of active substance present in a preparation produced therefrom may be less than the specified amount.
Another aspect which is important in the production of solid preparations is that the active substance should have the most stable possible crystalline morphology for pharmaceutical quality. If this is not the case, the morphology of the active substance may change in certain circumstances under the conditions of manufacture of the preparation. Such a change may in turn affect the reproducibility of the manufacturing process and thus lead to final formulations which do not meet the high quality requirements imposed on formulations of pharmaceutical compositions. To this extent it should be generally borne in mind that any change to the solid state of a pharmaceutical composition which can improve its physical and chemical stability gives a significant advantage of less stable forms of the same drug.
Thus, a further object of the invention is to provide a new, stable, crystalline form of telmisartan which complies with the abovementioned stringent requirements imposed on pharmaceutically active substances.
Surprisingly, it has been found that telmisartan can be obtained in crystalline form, as the sodium salt of the below formula 1. 
In accordance with conventions respecting the use of nonproprietary names, the telmisartan salt of formula 1 may be referred to as telmisartan sodium. Therefore, as used herein, the term xe2x80x9ctelmisartan sodiumxe2x80x9d is defined to mean the telmisartan salt of formula 1.
By a suitable choice of manufacturing conditions, the polymorphic form of the crystalline sodium salt which meets the requirements mentioned above can be obtained selectively.
This crystalline form of the sodium salt of telmisartan is characterised by having a melting point of T=245xc2x15xc2x0 C. (determined by DSC=Differential Scanning Calorimetry; heating rate: 10 K/min).
The present invention therefore relates to a crystalline form of telmisartan sodium that is characterised by having a melting point of T=245xc2x15xc2x0 C. (determined by DSC). The above value was obtained using a DSC821 made by Messrs Mettler-Toledo.
The crystalline form of telmisartan sodium according to the invention was examined more closely by x-ray powder diffraction. The X-ray powder diagram obtained is shown in FIG. 1.
The following Table 1 summarizes the data obtained in this x-ray powder diffraction analysis:
In the above Table the value xe2x80x9c2"THgr"[xc2x0]xe2x80x9d denotes the angle of diffraction in degrees and the value xe2x80x9cd [xc3x85]xe2x80x9d denotes the lattice plane spacings determined in xc3x85.
According to the findings given in Table 1, the crystalline form of telmisartan sodium that constitutes the invention is characterised in that, when subjected to analysis by x-ray powder diffraction, it exhibits a characteristic set of d-spacings that includes values at d 20.95 xc3x85, 17.72 xc3x85, 13.97 xc3x85 and 13.63 xc3x85.
The X-ray powder diagrams were recorded within the scope of the present invention using a Bruker D8 Advanced with an SSD (=site-sensitive detector) (CuKxcex1xe2x80x94radiation, xcex=1.5418 xc3x85, 30 kV, 40 mA).
The present invention also comprises the solvates and hydrates of the above-described crystalline form of telmisartan sodium, especially the hydrates, most especially the hemihydrate thereof.
In another aspect, the present invention comprises a method of producing the crystalline form of telmisartan sodium according to the invention. The starting material used to prepare the crystalline telmisartan sodium according to the invention may be the free acid of telmisartan, which may be obtained by methods known in the art (e.g. according to EP 502314 A1 and U.S. Pat. No. 5,591,762).
To prepare the crystalline telmisartan sodium according to the invention the free acid of telmisartan is taken up in a suitable solvent, preferably in an organic aprotic solvent, most preferably in an organic, aprotic and non-polar solvent. The solvents used according to the invention are most preferably toluene, chloroform, dichloromethane, tetrahydrofuran, diethylether, diisopropylether, methyl-tert, butylether, acetone, methylisobutylketone, benzene or acetonitrile, of which toluene, benzene and methylisobutylketone are particularly preferred. Of outstanding importance according to the invention is toluene as solvent.
Preferably, between 0.5 and 5 ml, more preferably between 1 and 3 ml, most preferably between 1.5 and 2.5 ml of the abovementioned solvent are used per gram of telmisartan (free acid).
A suitable sodium salt is then added as a base to this solution or suspension. Suitable sodium salts within the scope of the present invention include sodium hydroxide, sodium hydride, sodium carbonate, sodium hydrogen carbonate or sodium alkoxides. By sodium alkoxides are meant the sodium salts which are formed with lower alcohols, preferably with alcohols selected from among methanol, ethanol, isopropanol, n-propanol, tert-butanol, sec.-butanol, isobutanol, n-butanol and tert.-amylalcohol. Of particular interest according to the invention are sodium salts selected from among sodium hydroxide, sodium hydride, sodium ethoxide and sodium methoxide; of these, sodium hydroxide and sodium methoxide are of particular importance according to the invention. The abovementioned sodium salts may be added to the reaction mixture as solids. In the case of sodium hydroxide this is preferably added in the form of aqueous solutions, however. It is particularly preferable to use concentrated aqueous solutions of sodium hydroxide.
For example, sodium hydroxide solution may be used in a concentration of about 45 wt.-%
The amount of sodium salt to be used naturally depends on the amount of free acid telmisartan used. According to the invention at least 1 mol of sodium salt has to be added per mol of telmisartan. It is also possible according to the invention to add an excess of sodium salt. Preferably, 1-2.5, more preferably 1-2, most preferably 1-1.5 mol of sodium salt are added per mol of the acid telmisartan used.
If sodium hydroxide is used as the sodium salt and this is added in the form of an aqueous solution, according to a preferred embodiment of the process according to the invention, it may be helpful in some cases to add a water-miscible organic solvent. This is preferably selected from among methanol, ethanol, isopropanol, acetone, tetrahydrofuran, tert.-butanol, 2-butanol, butanol, glycol, ethyldiglycol, 1,3-butanediol, 1,4-butanediol, tert.-amylalcohol, acetonitrile, nitromethane, formamide, dimethylformamide, N-methylpyrrolidinone, dimethylsulphoxide, dimethylacetamide, nitroethane and methoxy-2-propanol, of which the abovementioned alcohols are particularly significant. It is particularly preferred, within the scope of the process according to the invention, to use methanol or ethanol, most preferably ethanol. Preferably, between 50 and 500 ml, more preferably between 100 and 400 ml, most preferably between 200 and 350 ml of this solvent are used per mol of telmisartan used, according to the invention.
Then the reaction mixture may be heated to speed up the progress of the reaction. Preferably, the reaction mixture is heated to a temperature of  greater than 40xc2x0 C., most preferably to over 60xc2x0 C., with thorough mixing. The maximum temperature which may be selected is naturally determined by the boiling temperature of the solvents used. If the preferred solvents as described hereinbefore are used according to the invention, the mixture is preferably heated to over 70xc2x0 C. This heating is generally carried out for a period of from 15 minutes to 2 hours, preferably between 20 minutes and one hour. Then the solution obtained is filtered and any solid remaining in the filter is washed with one or more of the abovementioned solvents.
The filtrate obtained by the process described above is added slowly, preferably dropwise, to an organic solvent which is heated to a temperature of  greater than 40xc2x0 C., preferably above 60xc2x0 C., most preferably to boiling point. The solvent used is preferably an organic aprotic solvent, more preferably an organic, aprotic and non-polar solvent. Solvents which may be used according to the invention are, most preferably, toluene, chloroform, dichloromethane, tetrahydrofuran, diethylether, diisopropylether, methyl-tert. butylether, acetone, methylisobutylketone, benzene or acetonitrile, of which toluene, benzene and methylisobutylketone are particularly preferred. The solvent toluene is of exceptional importance according to the invention. At the same time as the filtrate is added to the heated solvent, in a preferred embodiment of the invention, some of the solvent is distilled off (optionally azeotropically). After all the filtrate has been added, more solvent (e.g. about one to two thirds of the total amount of solvent added by this stage) may optionally be removed by distillation.
The concentrated solution thus obtained is cooled, preferably to ambient temperature, whereupon the telmisartan sodium salt crystallises out. After crystallisation is complete the crystals are separated off, optionally washed with the organic solvent mentioned above and finally dried.
In another embodiment of the invention the crystalline telmisartan sodium salt according to the invention may be obtained starting from the acid addition salts of formula 2
wherein Hxe2x80x94X denotes an acid selected from among hydrochloric acid, hydrobromic acid, toluenesulphonic acid or methanesulphonic acid. Of the abovementioned acid addition salts of formula 2 the salt wherein Hxe2x80x94X denotes hydrochloric acid is of particular significance. This acid addition salts is also referred to hereinafter as telmisartan hydrochloride.
The compounds of formula 2 may be obtained for example from tert.-butyl 4xe2x80x2-[[2-n-propyl-4-methyl-6-(1-methylbenzimidazol-2-yl)-benzimidazol-1-yl]-methyl]-biphenyl-2-carboxylate (=tert.-butyl ester of telmisartan) known from the prior art by saponification in acetic acid in the presence of the acid Hxe2x80x94X.
In order to prepare the crystalline telmisartan sodium salt of formula I according to the invention starting from the acid addition salts of formula 2 the following procedure may be used, according to the invention.
The compound of formula 2 is taken up in a suitable solvent and combined with a suitable sodium salt.
The solvent may be water and/or a suitable alcohol, such as methanol, ethanol or isopropanol mixed with an aprotic organic solvent selected from among toluene, chloroform, dichloromethane, tetrahydrofuran, diethylether, diisopropylether, methyl-tert. butylether, acetone, methylisobutylketone, benzene and acetonitrile. It is particularly preferred to use, as the solvent, water mixed with ethanol or isopropanol mixed with an aprotic organic solvent selected from among toluene, benzene and methylisobutylketone, most preferably toluene. A mixture of water, isopropanol and toluene has proved particularly suitable for this step of the synthesis.
The amount of solvent or solvent mixture used depends on the amount of acid addition salt 2 used. Preferably, about 0.3-3.5 L, preferably about 1-2.5 L, more preferably about 1.5-2 L of the abovementioned solvent or solvent mixture are used per mol of compound 2 used. If the solvent used is the preferred solvent mixture according to the invention which contains an alcohol as the third solvent component in addition to water and an aprotic organic solvent, the ratios by volume of water to aprotic organic solvent according to the invention are preferably in a range from 1:5 to 1:50 and the ratio of water to alcohol used is in a range from 2:1 to 1:40. Preferably, in a solvent mixture of this kind, the ratios of water to aprotic organic solvent are in the range from 1:10 to 1:30, preferably in the range from 1:15 to 1:25 and the ratio of water to alcohol used is in a range from 1:1 to 1:20, preferably in the range from 1:5 to 1:15.
Preferably, the solvent or solvent mixture mentioned above contains about 10 to 100 ml of water, preferably about 30 to 80 ml of water, most preferably about 40 to 70 ml of water, per mol of 2. Preferably the solvent or solvent mixture used also contains about 100 to 1000 ml of alcohol, preferably about 300 to 800 ml alcohol, most preferably about 400 to 700 ml alcohol, per mol of 2. Finally, the solvent or solvent mixture used preferably contains as the third component of the solvent, about 200 to 2000 ml of the abovementioned aprotic organic solvent, preferably about 600 to 1600 ml, most preferably about 800 to 1400 ml of the abovementioned aprotic organic solvent, per mol of 2.
Suitable sodium salts which may be used for reacting 2 to 1 include sodium hydroxide, sodium hydride, sodium carbonate, sodium hydrogen carbonate or sodium alkoxides. By sodium alkoxides are meant the sodium salts which are formed with lower alcohols, preferably with alcohols selected from among methanol, ethanol, isopropanol, n-propanol, tert-butanol, sec.-butanol, isobutanol, n-butanol and tert.-amylalcohol. Of particular interest according to the invention are sodium salts selected from among sodium hydroxide, sodium hydride, sodium ethoxide and sodium methoxide, while the sodium alkoxides sodium ethoxide and sodium methoxide, particularly sodium methoxide are of particular importance according to the invention for this reaction step. The abovementioned sodium salts may be added to the reaction mixture as solids. In the case of sodium methoxide however it is preferable to add it in the form of a methanolic solution. Methanolic solutions of sodium methoxide which contain it in a concentration of at least 10%, most preferably about 20-40% (w/w), are particularly preferred. For example, the methanolic sodium methoxide solution used may have a concentration of about 30 wt. %.
The amount of sodium salt to be used is naturally dependent on the amount of free acid telmisartan used. According to the invention, at least 2 mol of sodium salt have to be added per mol of telmisartan acid addition salt of formula 2 used. According to the invention it is also possible to add an excess of sodium salt. It may be useful in some cases to add activated charcoal to the abovementioned reaction mixture. For example, it may be added in an amount of about 5-50 g per mol of 2 used, preferably in an amount of about 10-40 g per mol of 2 used. After the sodium salt and optionally the activated charcoal has been added the reaction mixture obtained is heated to a temperature of about 50-100xc2x0 C., preferably about 60-90xc2x0 C., most preferably about 70-80xc2x0 C. for a period of about 10 minutes to 2 hours, preferably for about 20-45 minutes. In the course of this heating, some of the solvent, preferably about 10-50%, most preferably about 20-40% of the total quantity of solvent may be distilled off. The remaining suspension is then filtered, the filter residue is optionally washed with one of the abovementioned aprotic organic solvents, preferably with the aprotic organic solvent which is also used in the reaction. The filtrate obtained is then diluted with a solvent or mixture of solvents. It is preferable to use a mixture of water and the abovementioned aprotic organic solvent for this. Preferably, at this point, about 10 to 100 ml of water, preferably about 30 to 80 ml of water, most preferably about 40 to 70 ml of water are used per mol of the compound 2 originally used. At this point, 250 to 3000 ml, preferably about 800 to 2000 ml, most preferably about 1200 to 1800 ml of aprotic organic solvent are used per mole of the compound 2 originally used. After dilution, the mixture obtained is refluxed. Then about 1-2 L, preferably about 1200 to 1800 ml of solvent are distilled off per mole of the compound 2 originally used. After the solvent has been distilled off the telmisartan-sodium salt 1 according to the invention crystallises out. The crystals obtained are isolated, optionally washed with one of the abovementioned aprotic organic solvents and then dried.
In another aspect the present invention relates to crystalline telmisartan-sodium salt, optionally in the form of the solvates or hydrates thereof, preferably in the form of the hydrates thereof, most preferably in the form of the hemihydrate, which may be obtained by the methods described above.
Because of the central significance of the compounds of formula 2 as valuable starting materials for the direct synthesis of the telmisartan-sodium salt 1 according to the invention, in another aspect the present invention relates to compounds of formula 2 per se 
wherein Hxe2x80x94X denotes an acid selected from among hydrochloric acid, hydrobromic acid, toluenesulphonic acid or methanesulphonic acid. The compound of formula 2 wherein Hxe2x80x94X denotes hydrogen chloride, the telmisartan hydrochloride, is particularly preferred.
Most preferably, the present invention further relates to the abovementioned compounds of formula 2 in crystalline form.
Moreover, in view of the pharmaceutical activity of the crystalline telmisartan sodium salt according to the invention, the present invention relates to the use thereof as a pharmaceutical composition.
In another aspect, in view of the pharmaceutical activity of the crystalline telmisartan sodium salt according to the invention, the present invention relates to the use thereof for preparing a pharmaceutical composition, particularly for preparing a pharmaceutical composition for the prevention or treatment of diseases wherein the administration of therapeutically effective doses of one or more angiotensin-II-antagonists may provide a therapeutic benefit. Preferably, the present invention relates to the use of crystalline telmisartan-sodium salt for preparing a pharmaceutical composition for the prevention or treatment of diseases selected from among hypertension, cardiac insufficiency, ischaemic peripheral circulatory disorders, myocardial ischaemia (angina), the progression of cardiac insufficiency after myocardial infarct, diabetic neuropathy, glaucoma, gastrointestinal diseases and bladder diseases, the prevention or treatment of hypertension being particularly preferred.
Accordingly, in another aspect, the present invention is directed to pharmaceutical formulations characterised in that they contain crystalline telmisartan-sodium salt.
The example of synthesis that follows serves to illustrate a method of preparing crystalline telmisartan-sodium salt carried out by way of example. It is intended solely as a possible procedure provided by way of example, without restricting the invention to its contents.